1. Field of the Invention
The present invention relates to synthetic aperture radar (SAR). More particularly, the invention relates to a sampling method and apparatus for imaging fast-moving surface displacements in real time, such as seismic waves, utilizing both single-pass and repeat-pass synthetic aperture radar interferometry (InSAR).
2. State of Technology
Seismology plays a dominant role in monitoring global natural (e.g., earthquakes) and man-made activity (e.g., underground mining). Seismology, however, is an inherently blind methodology, whereby the seismic waves are recorded by the effect they have on instruments as the seismic waves pass by. In addition, the type and quality of information available from seismology has been and continues to be limited by the relatively sparse spatial sampling on the earth's land surface and nearly complete lack of spatial sampling over the oceans. If these dynamic waves can be imaged in real time, actual movies of these waves can be produced yielding more information about their source and the structure they propagate through.
InSAR (Interferometric Synthetic Aperture Radar) is a new geodetic standard providing spatially continuous maps of surface displacements to the millimeter level, with 25-meter horizontal (pixel) resolution. Operational benefits of InSAR include all weather, day or night, spatially continuous, wide area (100 by 100 km image frames) and global coverage from space. While InSAR has predominantly been used to map and measure static deformations, the possibility exists for applying InSAR techniques to image dynamic deformations, such as those from seismic waves propagating along the earth's surface.
Three typical ways of acquiring InSAR data are: 1) cross-track single pass (dual antenna), 2) cross-track repeat pass (single antenna), and 3) along-track single pass (dual antenna). The single-pass methods require two SAR antennas mounted on the same platform for simultaneous acquisition. The repeat-pass method requires only one antenna that acquires data over the same area twice via a repeat pass, but with a slightly different viewing geometry. This approach requires precise knowledge and predictability of the flight path and hence it is best suited to space-borne systems.
Along-track interferometry (ATI), which is applicable to the remote measurement of velocities of objects moving on the ground, images a ground region twice at slightly different times by producing two SAR images. The movement of the measured object in the images, e.g., water currents, causes a phase shift between the corresponding pixels in the SAR images. The moving surface produces a Doppler shift relative to the other stationary ground pixels,. A Doppler image can thus be created whereby only moving objects are visible in the image. Measurements from an inertial navigation system may be used to correct for unwanted phase shifts caused by aircraft yaw, pitch and roll. While a SAR is usually operated as a single-channel SAR, its antenna may be divided in the direction of movement and operated as a two-channel ATI SAR (i.e., an along-track interferometer).
Background information on a single SAR line utilizing a single pass is disclosed in U.S. Pat. No. 5,945,937, titled “ALONG-TRACK INTERFEROMETRIC SYNTHETIC APERTURE RADAR,” issued Aug. 31, 1999, to Fujimura, including the following: “An along-track interferometric SAR of the present invention includes a single SAR channel for acquiring SAR data by observing a target only once. A reconstructing section reconstructs two SAR images deviated in time from the SAR data to thereby output two reconstructed SAR images. A detecting section calculates a phase difference between the two reconstructed SAR images to thereby detect the velocity of the target in the SAR eye direction. The reconstructing section bisects, based on the fact that the frequency of a wave returned from the ground undergoes a Doppler shift due to the movement of a SAR relative to the ground, the SAR data in the frequency domain and processes the bisected SAR data.”
Despite existing conventional systems and methods, a need exists for an improved dynamic InSAR for real time assessment of surface displacements, such as, but not limited to, seismic waves. The present system and method is directed to such a need.